In implantable cardioverter defibrillators (ICDs) used to treat cardiac arrhythmias, a cardiac arrhythmia therapy is automatically classified by the ICD as successful if normal sinus rhythm (NSR) is detected after therapy delivery. Conversely, the therapy may be declared unsuccessful if the arrhythmia is re-detected following completion of the therapy. However, in some cases, the normal sinus rhythm may occur later than an effective time period for the therapy. A heart rhythm may remain unclassified after therapy delivery during a period of time in which detection criteria for detecting normal sinus rhythm and detection criteria for redetecting an arrhythmia both remain unmet. An eventual NSR classification may result in the therapy outcome being classified as successful even though the restored NSR occurred later than the expected effective time period of the therapy. Such a successful therapy outcome classification overestimates the therapy success rate since the return to sinus rhythm is more likely representative of a self-terminating arrhythmia then the result of an effective therapy.